The present invention relates to a method and apparatus for removing droplets of liquid entrained in a gas stream.
There are a number of industrial processes wherein a liquid and a gas come into direct contact with each other as a part of the process. As a result of viscous and aerodynamic forces, liquid droplets of various sizes are entrained and carried along with the moving gas stream. In most instances it is desirable or even mandatory that these droplets be removed from the gas stream for economic or environmental reasons. This invention provides a highly efficient means for collecting droplet mists in a gas stream over a wide range of droplet sizes and gas velocities, and the droplet removal task is accomplished with low energy consumption. This device, commonly referred to as a mist eliminator or drift eliminator, is adaptable to direct contact processes employing either counterflow or crossflow liquid and gas flow relationships and is particularily suitable for removing water droplets from an air stream as required in cooling towers of either the counterflow or crossflow type.
Mist eliminators known heretofore have certain performance and application limitations which are overcome in the present invention. There are five primary considerations in the design and application of mist elimination devices. First and foremost is the requirement of a high collection efficiency in the removal of liquid droplets. Without a high collection efficiency over the full liquid droplet size range encountered, the device has limited application. Most known mist eliminators have high droplet collection efficiencies for droplet sizes of 100 microns or greater in the gas velocity range of current application practice. However, in droplet sizes below 100 microns droplet collection efficiencies fall off rapidly. In many processes the majority of entrained droplets are found in the smaller size ranges. When high collection efficiencies for smaller droplet size ranges are required with known mist eliminators it becomes necessary to take additional steps or to restrict operating limits to remove these smaller droplets. Common means for doing this in current practice are to add secondary droplet removal devices, to limit gas flow velocities, to increase the density of mist removal components, to utilize mist eliminator designs with higher energy consumption or some combination of the above. The mist eliminator of the present invention has a substantially higher droplet collection efficiency particularly in the smaller droplet sizes over a wider range of gas velocities than heretofore known thereby eliminating the need for employing a secondary removal means or limiting operational parameters either of which result in higher equipment first cost and higher energy consumption.
The second consideration in mist eliminator design is to minimize the resistance to gas flow through the device. A high gas flow resistance per unit area will result in high energy consumption and will normally require larger gas moving equipment or an increase in the size of the process equipment itself. The present invention has air resistance characteristics similar to known low energy loss mist eliminators but achieves higher collection efficiencies and stable operation over a wider range of velocities than known mist eliminators.
A third design consideration is to assure that liquid collected on mist eliminator surfaces will not be torn from these surfaces at critical points by viscous and aerodynamic forces imposed by gas flow through the device thereby causing carry over and liquid re-entrainment which negates the purpose and effectiveness of the mist eliminator. This phenomenon is primarily a function of mist eliminator design characteristics and gas flow rate. The gas velocity at which carry over and re-entrainment become a significant problem may be described as the breakdown velocity. The present invention has a breakdown velocity from 20 to 80 percent higher than known mist eliminator designs thereby allowing higher gas flow rates through the mist eliminator while maintaining high droplet collection efficiencies and assuring stable operation.
The fourth design consideration is in providing an adequate means for draining collected liquid from the mist eliminator such that liquid does not build up on the mist eliminator surfaces thereby restricting gas flow or creating a carry over problem as previously described. In mist eliminators employing meshes or screens or those that use a bed filled with beads or rings, etc., this can be a major design limitation. In blade type designs such as the present invention, drainage is not usually a major concern. Collected liquid will naturally adhere to the blade surface and will drain as large drops or streams ultimately flowing by gravity to the liquid means of the process apparatus. Liquid flow may be directed to a specific point which is shielded from the main gas flow stream by inclining blades or the entire mist eliminator assembly at an angle relative to a horizontal plane. This is easily accomplished with the present invention in assembly or installation.
The final design consideration relates to structural integrety, dimensional stability and ease of assembly and installation. The mist eliminator design of the present invention incorporates a simple and unique mechanical interlocking means for connecting blades and end plates which allows mist eliminators to be assembled into panels. It also assures dimensional accuracy and consistency during and after assembly and provides a positive blade end seal in panel assemblies.
Illustrations of the principal prior mist eliminator designs and a newly proposed design are presented in a very recent article entitled "Design Studies of Current Design and Improved Cooling Tower Drift Eliminators" by Joseph K. Chan and Michael W. Golay, presented at the Symposium on Environmental Effects of Cooling Tower Emissions, at University of Maryland, May, 1978, Proceedings III pages 127-149. That article illustrates the (a) sinus-shaped, (b) HI-V, (c) zig-zag, (d) single-layer louver, (e) double-layer louver, and (f) asbestos-cement drift eliminator geometries in FIG. 1 and a recent airfoil drift eliminator geometry in FIG. 15. The specific construction of a geometry referred to as the "HI-V" geometry is illustrated in U.S. Pat. No. 3,748,832 to D. B. Furlong and J. C. Ovard which is assigned to Fluor Cooling Products Company.